1. Field of the Invention
The present invention relates to a wideband antenna, and more particularly, to a wideband antenna for generating resonance effect via coupling feed-in and direct feed-in methods, so as to combine a wideband characteristic of the coupling feed-in method and a well matching characteristic of the direct feed-in method, to improve high-frequency bandwidth and low-frequency matching simultaneously.
2. Description of the Prior Art
An electronic product having a communication function, such as a laptop computer, a personal digital assistant, etc., uses an antenna to transmit or receive radio waves, so as to transmit or receive radio signals, and access wireless network. Therefore, in order to let a user to access wireless network more conveniently, a bandwidth of an ideal antenna should be extended as broadly as possible within a tolerable range, while a size thereof should be minimized as much as possible, to meet a main stream of reducing a size of the electronic product.
Planar Inverted-F Antenna (PIFA) is an antenna commonly used in a radio transceiver device. As implied in the name, a shape of PIFA is similar to an inverted and rotated “F”. PIFA has advantages of low production cost, high radiation efficiency, easily realizing multi-channel operations, etc. However, a bandwidth of PIFA is limited. Thus, in order to improve this disadvantage, the applicant of the present invention has provided a dualband antenna 10 shown in FIG. 1A in U.S. Pat. No. 7,602,341. Comparing to a traditional dualband antenna, the dualband antenna 10 adds a radiation part 12 for providing an extra high frequency resonance mode, such that a high frequency band of the dualband antenna 10 is composed of two resonance modes. FIG. 1B illustrates a schematic diagram of voltage to stand wave ratio (VSWR) of the dualband antenna 10. If the dualband antenna 10 does not add the radiation part 12, the dualband antenna 10 becomes a dualband antenna 20 shown in FIG. 2A. A high frequency bandwidth of the dualband antenna 20 reduces substantially and VSWR of the dualband antenna 20 is shown in FIG. 2B. From the above, the dualband antenna 10 effectively increases the high frequency bandwidth with the two resonance modes. However, the dualband antenna 10 is not suitable for some applications and may affect the antenna characteristic if one of the resonance modes suffers from insufficient bandwidth or frequency shift.